1. The Field of the Invention
The present invention relates to electrical connections. More particularly, the present invention relates to printed circuit boards and electrical grounding thereof. In particular, the present invention relates to a cover-to-cover grounding contact clip in an electronic peripheral card for the control and management of electromagnetic interference and electrostatic discharge.
2. The Relevant Technology
The proliferation of personal computers has caused a large demand for expansion boards to enhance computer functionality. With older personal computers, expansion boards were mounted internally and therefore required the computer cover to be removed before installation thereof. The advent of electronic devices such as laptop and notebook personal computers, handheld computers, and other devices such as personal digital assistants (PDAs) necessitated the development of a portable expansion board. Such a portable expansion board has been referred to as a peripheral card with one standard configuration being a PCMCIA card. PCMCIA cards are insertable in external slots of laptops, notebooks, sub-notebooks, and other hand-held devices such as PDAs. The small size of these electronic devices and their portability facilitates the interchange of functionality by installing and removing different peripheral cards into the external slots of the computer or other electronic device. Typical peripheral cards include modems, wireless communicators, and memory expansion inserts.
Because these peripheral cards may be removed from the electronic device, both their installation and their operation within the electronic device may trigger electrostatic discharge (ESD) or also electromagnetic interference (EMI) during operation. An electrostatic charge may build-up on any conductive surface of a peripheral card and as such, ESD may cause damage to both the electronic components within the peripheral card as well as the electronic device into which the peripheral card is inserted. An ESD therefore must be properly managed for such peripheral cards. An effective discharge path to a ground source, such as a computer chassis, is needed from any major surface area of the card.
Another desirable design goal is that the peripheral card be adequately shielded against radiating or receiving EMI. The shield must have a sufficiently low ohmic resistivity such that no anticipated electromagnetic energy can penetrate it. The shield therefore must have the function of both adequate thickness and adequate ohmic resistance. Currently, peripheral cards such as PCMCIA cards are constructed with a relatively thin two-sheet, metal shield that can provide adequate shielding at all anticipated frequencies. The shield also needs to provide a conductive surface area in order to allow for a substantially continuous current flow through the shield surface to ground. If a discontinuity arises between the shield and the leads to ground, a slot antenna is created and EMI can penetrate the peripheral card or it can be generated by the peripheral card such that it interferes with the larger electronic device.
Several packaging methods have been developed for the construction of peripheral cards that assist to provide ESD and EMI protection. When an insufficient electrical conduction path to ground exists, many problems may arise during operation of the peripheral card and it may also cause problems during operation of the electronic device with which the peripheral card is connected. To overcome EMI problems, it is therefore preferable to increase the ground-to-signal ratio in a PCB to improve the signal flow therethrough which enhances signal transmission performance of the peripheral card. Additionally, where a PCB does not have proper connection to ground, the PCB can suffer from not only EMI interference problems, but also ESD problems which may arise due to the live electronic components on the peripheral card which may arc to the card""s metallic shield.
ESD may also occur when two device surfaces exhibit a sufficient difference in electrical potential resulting from varying electrical charge on each surface. Such a condition forms an electrostatic capacitor between the two devices which emits electrostatic electrons that may also interfere with the performance of the related devices and other peripherals. When these charged surfaces abruptly encounter each other, such as when an electrical card and a related card connector are coupled together, an electrostatic discharge may result thereby causing damage to the components within the electrical card.
Several arrangements have been made in the art to provide adequate PCB-to-ground paths for peripheral cards. One example is a compression spring that is constrained in a pocket of the package frame for extendable portions of a peripheral card. The compression spring must be fitted onto the frame pocket by hand, it is small, and because it is a separate external piece, it may be easily lost.
Another example is a clip that is soldered to one side of the peripheral card""s PCB. The soldered clip has the problem that it requires hand-soldering and handling and that a multiple number of clips may be required to accomplish a cover-to-cover contact for the peripheral card.
Another article that has been used to deal with both EMI and ESD is a clip that is crimped to the plastic frame of a peripheral card such as a PCMCIA card and that makes electrical contact with both the upper metallic sheet and the lower metallic sheet. Additionally, a portion of the clip has a tab that extends therefrom and makes electrical contact with an electrically conductive pad on the PCB. The crimped clip, because of its extremely thin nature, can become damaged due to conventional handling during assembly. Additionally, the tab, because it extends beyond the frame, may require intensive labor to install and to crimp. Additionally, the clip cannot be reworked and the tab may not make adequate contact with the pad on the PCB. Additionally, there are multiple steps of manufacture and assembly which are wasted if a subsequent process causes damage.
Another device that has been used to manage EMI and ESD is a formed tab that is integral to the metallic shield. As with the clip on the PCB and tab in the crimped clip, the formed tab can be easily damaged during assembly and may not make adequate contact with the PCB. It is also flimsy and potentially unreliable because its resilience may be damaged.
Another article that has been used for EMI and ESD management is a gasket material that is compressed between one of the metallic shields and the PCB. The gasket material, however, may not remain intact or in its proper orientation. As such, it may cause irreparable damage to electronic components contained in the peripheral card and it requires a great deal of handling to assemble.
As with all of these prior art attempts for EMI and ESD protection, where proper contact is not made, the peripheral card can become susceptible to ESD or act as an antenna and/or create unwanted electromagnetic emissions. Additionally, if a metal shielded connector with contact tabs is used as part of the peripheral card, and the contact tabs do not make proper contact, the peripheral card can become an antenna.
Another configuration that contains both EMI and ESD is the placement of a ground trace around the outer perimeter of a PCMCIA card, followed by the overmolding of the PCB with a first dielectric material, and a second conductive material with the placement of a conductive adhesive and a metallic sheet upon the adhesive material. The packaging of a PCMCIA card after this fashion allows for the dissipation of both EMI and ESD. This configuration has several drawbacks including (i) a substantially permanent sealing off of the PCB surface, (ii) the overmolding""s blanket effect that prevents cooling of the PCB components, (iii) poor electrical conductivity through the electrically conductive adhesive and (iv) a very large portion of the card""s surface area occupied by the ground trace.
FIG. 1 depicts yet another configuration for providing electrical continuity between the printed wiring board and the shielding covers surrounding a module. In FIG. 1, an exploded elevational view of typical components in a prior art peripheral card such as a PCMCIA card are shown. As can be seen, a PCMCIA assembly 10 includes an upper or first metallic sheet or cover 12, a printed wiring or circuit board 14, a connector such as a multiple-socket connector 16 (typically used for interfacing with a host device or computer) that attaches to one end of circuit board 14, and a lower or second metallic sheet or cover 20. Assembly 10 further comprises a connector 22 for interfacing with a communication network and is depicted as a retractable connector. Assembly 10 is further comprised of a connector 18 for generally providing an additional digital interface to circuit board 14. In several implementations, connector 18 provides a cellular or wireless interface for coupling with wireless transceivers or other digitally-interfacable devices.
To simplify manufacturing of assembly 10, connector 18 is encompassed by a shielding cover 24 which includes shielding cover contact tabs 26 that provide conductive electrical contact with top or first cover 12 through a spring expansion tab contact interface. Connector 18 further includes a shielding circuit board connection tab 28 for electrically interfacing with a connector shielding pad 30 on circuit board 14. This configuration provides electrical contact between the top cover 12 and the circuit board""s ground structure. Connector 24 includes tabs symmetrical to tabs 26 on the lower side for electrically interfacing with bottom or second cover 20 thereby electrically coupling the ground plane of circuit board 14 with both top and bottom covers 12, 20.
As the miniaturization increases for peripheral devices, the xe2x80x9creal estatexe2x80x9d comprising the available surface on both sides of the PCB becomes more valuable, and structures that suppress both EMI and ESD occupy needed room on the PCB surface. Furthermore, as commonality between successive product assemblies is extended, it is desirable for manufactures to design fewer unique electrical circuit boards for spanning related product lines. Therefore, manufactures may vary configurations of designs by deleting or replacing components without entirely redesigning electronic circuits and mechanical assemblies.
Other problems arise when clips or other connections are made at the periphery of the PCB. For example, clips or tabs placed on the edge of the PCB exert forces on the sidewalls of the module covers which in turn induce rotational motion in the PCB. Such motion causes the PCB and connectors attached thereon to slide within the external enclosure that provides the physical alignment of the module with the computer. Such misalignment of the connectors of the PCB with the receiving computer results in a glancing contact and may even result in damage to connector contact pins and sockets.
Additionally, due to manufacturing robotics and automated processes as well as the miniaturization of electronic circuits and their components, it would be an advancement in the art to provide a shielding coupling system that could be robotically place and processed in a manner consistent with the other components of an electronic assembly without requiring non-standard or even manual assembly.
Thus, what is needed in the art is a structure for an assembly that addresses the ESD and EMI concerns in those assemblies and, further, can be subjected to similar assembly and manufacturing processes as the assemblies traditional components.
The present invention relates to an electrostatic (ESD) surface-mount clip for electrically coupling a printed circuit board (PCB) with opposing conductive covers in a module assembly such as a PCMCIA card. The ESD surface-mount clip is an article of manufacture that includes an integrally conductive profile for providing electrical continuity between a ground pad on the PCB and the opposing covers. The ESD surfacemount clip is comprised of a generally planar electrically conductive spanning means for spanning a cut-out or other opening on a printed circuit board that facilitates electrically coupling from one conductive cover of an electronic assembly to the opposing side or cover of the electronic assembly. The clip further includes surface mount pad that are integrally attached or formed from the same conductive material. The pads facilitate the bonding of the clip with the printed circuit board. The spanning means is formed to include opposing tabs, one extending in a direction toward a first cover and another extending in a generally opposing direction to encounter the opposing or second cover. A spanning member generally couples between the opposing tabs to form support or electrical continuity therebetween.
The ESD surface-mount clip of the present invention finds applications in modules where an exposed contact pad, generally a grounding pad, is located near the periphery of a cut out region on the PCB. Such open and available ground pads are regularly present in module embodiments wherein particular components such as connectors that would otherwise provide the continuity mechanism are absent. Such absence of components is common in electronic component families that reuse standardized PCB designs over a variety of mutated products.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by the practice of the invention. The features and advantages of the invention may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims. These and other features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.